Most renal sarcomas are secondary involvement from a retroperitoneal origin.
Renal sarcomas arising primarily within the kidney are very rare.
Morphologic features are similar to their soft tissue counterparts.
Most common is leiomyosarcoma, which may arise from the renal veins and its branches (image A) & (image B).
Synovial sarcoma is a biphasic tumor composed mostly of mesenchymal spindle cells and focal epithelial cells; exhibits a specific chromosomal translocation t(X;18) and shows SYT-SSX2 gene fusion (soft tissue SS has SYT-SSX1 fusion).
Primitive neuroectodermal tumor (PNET) shows primitive round blue cells that are CD99+ and is characterized by t(11;22), which results in fusion of EWSR1 and FLI1 oncogene.
Angiosarcoma is exceedingly rare.
Liposarcoma may look like fat predominant angiomyolipoma and generally is a secondary extension from the retroperitoneum.
Main DDX for sarcoma with spindle cells:
Sarcomatoid RCC: Can be mixed with usual RCC morphology and is keratin+ (since it's an epithelial tumor); search for the usual epithelial RCC component to establish the diagnosis.